The efficacy of postemergence herbicide applications was evaluated in the greenhouse and the field in 1993 and 1994, using variables of carrier volume and diluent type. Ultra-low volume (ULV) applications of herbicides using an air-assisted sprayer calibrated at a volume of 9.4 L/ha were compared to 94 and 187 L/ha applications with a conventional hydraulic system. In greenhouse studies, reduced rates of sethoxydim, clethodim, fluazifop-P, and quizalofop were used to compare the effects of carrier types and volumes on their efficacy to yellow foxtail. All herbicides applied in oil diluents at 9.4 L/ha, with the exception of fluazifop-P and quizalofop applied in soybean oil, provided better yellow foxtail control than when applied in water at 94 and 187 L/ha. Sethoxydim and clethodim applied at 25 and 16% of the recommended rate, respectively, provided over 90% control in both petroleum oil and methylated seed oil. Reduced rates of glyphosate in petroleum oil or soybean oil were more effective at reducing bermudagrass growth than in water or methylated seed oil. Activity of glyphosate in methylated seed oil was no more effective than in water. Field experiments were conducted to study the efficacy of ULV applications of sethoxydim, clethodim, fluazifop-P, imazethapyr, and bentazon. Rates of the postemergence grass herbicides could be reduced by applying them at ULV in oil diluents while maintaining annual grass control equal to that achieved with higher herbicide rates in larger volumes of water. Control of annual grasses and annual broadleaf weeds with bentazon and imazethapyr applied at ULV were inconsistent and did not provide acceptable weed control. This may be partially due to the poor suspension of these herbicides in oil carriers.